Covering impregnated with insecticide

ABSTRACT

A covering for soft furnishing is impregnated with an insecticide, such as permethrin. The concentration of permethrin is sufficient to kill house dust mites and deter larger insects such as fleas and ticks. The insecticide is released from the covering over an operational period so as to encourage the absorption of the insecticide into surrounding soft furnishings. The covering is manufactured from machined fabric, such as knitted voile, having interstices sufficiently small to reduce the rate of insecticide release over the operational period.

FIELD OF THE INVENTION

[0001] The present invention relates to coverings impregnated withinsecticide for soft furnishings. In particular, the invention relatesto coverings impregnated with insecticide, wherein the insecticide isreleased from the covering over an operational period.

BACKGROUND OF THE INVENTION

[0002] Dust mites live in substantial numbers in places offering warm,humid conditions and a supply of human skin scales and other debris uponwhich they feed. The surface of a mattress is a particularly attractivehabitat and it is understood that the mites move significant distanceson and within mattresses in order to seek out and to remain withinoptimal environmental conditions. The World Health Organisation hasidentified dust mite concentrations at which genetically predisposedindividuals develop antibodies to the house dust mite allergen andconcentrations at which there are risks of acute asthma attacks inmite-allergic patients. Thus, it is possible to perform an examinationof soft furnishings, such as beds, used by an allergic individual todetermine concentrations of house dust mites and to take appropriateaction in order to reduce or eliminate these concentrations.

[0003] When a permethrin impregnated mattress cover is fitted to a bed,the permethrin formation comes into intimate contact with the surfaceand sides of the mattress. This close contact, together with thepressure of the occupant of the bed, causes some of the permethrin tomove onto the surface of the mattress itself and then subsequently intothe surface layers of the mattress. The texture of the mattress cover,in combination with movement between the cover and mattress, results inthe permethrin being distributed relatively uniformly within themattress itself. As the mites move within the mattress, most of themwill eventually come into contact with this relatively uniform andcontinuous zone of permethrin impregnated fabric resulting in theabsorption of the chemical into the mite's body.

[0004] Permethrin that has become absorbed or deposited on the mite'sfoodstuff may become ingested allowing it to enter into the haemolymph.In addition, while the mite is in physical contact with deposits ofpermethrin, small quantities of the chemical adhere to the miteresulting in diffusion onto the surface of the mite's cuticle. Once onthe cuticle, the permethrin diffuses directly through the cuticularlayers and thereafter passes into the mite's haemolymph. Once in thehaemolymph, the chemical becomes absorbed by the haemolymph lipoproteinsresulting in subsequent transportation to the site of action. Inaddition, permethrin may also diffuse laterally along the trachea of themite, thereby resulting in it being passed directly to the mite'snervous system.

[0005] The presence of permethrin within the mite's nervous systemresults in an increase in the extent to which sodium ions diffuse intothe axon of the nerves, resulting in depolarisation. This in turnresults in either a spontaneous nerve firing or in the blocking offurther action potentials passing along the nerve fibre. The end resultof these processes is a disruption in the passage of nervous impulsesalong the nerve causing either muscular excitation or paralysis,depending upon the stage of the intoxication process.

[0006] Disruption of coordinated muscle control does not by itself leaddirectly to the death of the mites but once this control has been lost,a finite period of time will elapse until an essential life sustainingprocess becomes sufficiently deranged to result in the death of themite. Thus, a common cause of mite death due to the presence ofpermethrin is that of failure to control water balance, eitherinternally or by an inability to seek a suitably humid environment.

[0007] Patent publication WO 96/28974 discloses the use of a nettingimpregnated with permethrin and fabricated into a cover appropriate forthe covering of domestic articles such as mattresses and duvets etc.Tests in the laboratory suggests that a covering fabricated from thenetting disclosed in the aforesaid patent publication should provideprotection for a significant period of time when impregnated with amodest quantity of insecticide. However, field trials, conducted undermore real life conditions, have shown that the duration over which a netcover provides adequate protection is significantly less than thatanticipated by use in the laboratory. Furthermore, tests of this typehave also shown that increasing the initial dose of insecticide does notsignificantly increase the active life of the covering in a proportionalway. Furthermore, there is a limit to the extent to which anyinsecticide (such as permethrin) could be used in a health caresituation.

[0008] Permethrin is not considered toxic to humans, howeverover-exposure may lead to increased hypersensitivity to touch and sound,tremors and convulsions. Contact with permethrin may produce skinsensations such as numbing, burning or tingling but these are reversibleand usually subside within a period of twelve hours. When placeddirectly in contact with the skin, tests have shown that about 0.5percent of the chemical is absorbed by the skin, therefore care shouldbe taken so as to minimise human exposure while maintaining thepesticide effect, as in any pesticide use.

[0009] It is an object of the present invention to increase theoperational period over which a covering impregnated with insecticide iseffective, without being required to increase initial insecticideconcentrations.

SUMMARY OF THE INVENTION

[0010] According to a first aspect of the present invention, there isprovided a covering for soft furnishing, impregnated with insecticidewherein said insecticide is released from said covering over anoperational period so as to encourage the absorption of said insecticideinto said soft furnishings, comprising a covering manufactured frommachined fabric having interstices sufficiently small to reduce the rateof insecticide release over said operational period.

[0011] In a preferred embodiment, the covering is configured to beapplied to mattresses, duvets, pillows or other bedding elements.

[0012] Preferably, the insecticide is a synthetic pyrethroid such asdeltamethrin, sypermethrin, allethrin or bifenthrin and in a preferredembodiment the synthetic pyrethroid is permethrin.

[0013] In a preferred embodiment, the fabric is machined from polyesteryam, preferably having a gauge in the range of twenty to one hundreddecitex. Preferably the interstices of the fabric are smaller than thatprovided by machined netting and in a preferred embodiment, the machinedfabric is a knitted voile. Preferably, the knitted voile has a weight ofbetween thirty-five to forty-five grams per square metre and containsbetween four hundred to seven hundred milligrams of insecticide(preferably permethrin) per square metre.

[0014] According to a second aspect of the present invention, there isprovided a covering for soft furnishings, impregnated with permethrinwith a loading of between five hundred and six hundred milligrams persquare metre and configured to release permethrin over an operationalperiod of between one-and-a-half and two-and-a-half years to give afinal loading of between one hundred to two hundred milligrams persquare metre.

[0015] Preferably, the covering is fabricated totally or in part ofknitted voile and said voile may be knitted from polyester yam.

[0016] According to a third aspect of the present invention, there isprovided a method of producing a covering for soft furnishingsconfigured to provide protection against house dust mites, comprisingthe steps of knitting a voile from a polyester yarn; machining a coverfrom said knitted voile; and impregnating said machined cover with aninsecticide.

[0017] In a preferred embodiment, the machined cover is impregnated bybeing immersed in an impregnation fluid and then rolled between rollersto control the level of fluid pick-up. Preferably, the rolled voile ispassed through drying means to remove water from the insecticide.

[0018] In a preferred embodiment, the impregnation fluid is a suspensionof permethrin in water and the impregnation fluid may be mixed by addingpermethrin in organic solution to water to provide the requiredsuspension. Preferably, the organic solution contains from five to fiftypercent permethrin and between fifty and ninety millimetres of organicsolution may be added to each litre of water.

[0019] Preferably, an insecticide loading of between four hundred andseven hundred milligrams per square metre is achieved by a wet pick-upof between fifty to seventy percent. Preferably, the insecticide ispermethrin.

BRIEF DESCRIPTION OF THE DRAWINGS

[0020]FIG. 1 shows a domestic environment including a number of articlesof soft furnishing, including a mattress;

[0021]FIG. 2 shows a diagrammatic cross-section of the mattressidentified in FIG. 1, surrounded by an impregnated cover;

[0022]FIG. 3 illustrates a knitted voile representing a preferredfabric;

[0023]FIGS. 4A, 4B and 4C illustrate rates of permethrin loss fordifferent types of material; and

[0024]FIG. 5 illustrates a permethrin impregnation process.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0025] The invention will now be described by way of example only withreference to the previously identified drawings.

[0026] A domestic environment shown in FIG. 1 includes a bed 11 having amattress 12 made up with a duvet 13 and pillows 14. The environment alsoincludes a carpet 15, a chair 16 and curtains 17. The chair 16 hascushions 18 and 19 applied thereto. All of these articles of softfurnishing, including the carpet 15 and the curtains 17 provide suitableenvironments for populations of house dust mites. However, the presenceof house dust mites are particularly annoying to allergenic suffererswhen found in mattress 12, duvet 13, pillow 14, cushion 18 or cushion19. Thus, in order to provide relief from these allergenic reactions, itis preferable to treat such items when an allergenic person wouldnormally come into sustained contact with such items, particularlymattress 12, duvet 13 and pillow 14.

[0027] Mattress 12 is detailed in FIG. 2. Mattress 12 is made up of acentral core offsprings 21 (or foam) surrounded by a layer of wadding22, both encapsulated in a textile covering 23. House dust mitepopulations are often found in the layer of wadding 23, behind thetextile cover 25. An insecticide impregnated covering 24 is placedaround the mattress so as to totally enclose the top and sides of themattress. After the mattress has been enclosed in this way, it may beused in a conventional fashion, possibly having a fitted sheet 25applied thereto.

[0028] If after covering a mattress with an impregnated sheet, symptomsstill persist, it may become necessary to take further action so as toreduce if not eliminate a sufferer's exposure to mite debris. Inparticular, similar covers may be placed over pillows, duvets andcushions etc. Preferably, the impregnated covering is applied directlyto the duvet, cushion or pillow whereafter conventional coverings may beapplied in the usual way. The objective is to bring the impregnatedcovering into a position which is as close as possible to the locationspreferred by the mites; usually within soft wadding material that isdisplaced from the centre of such material and towards the source ofheat, moisture and food created by the human occupants.

[0029] Previously, known impregnated coverings have been fabricated froma netting material, primarily because netting materials are familiar asinsect barriers. Thus, for example, netting is often used as a physicalbarrier for mosquitoes. Furthermore, a covering made from a nettingmaterial is relatively lightweight and its presence would not be obviouswhen positioned between, say, a mattress and a conventional mattresscovering or sheet. However, when impregnated with an insecticide, thematerial provides a different type of functionality to that provided bya mere physical barrier.

[0030] In the preferred embodiment, and as disclosed in internationalpatent publication WO 96/28974, a preferred insecticide for the purposeof eliminating house dust mites is permethrin. Permethrin C₂₁H₂₀CL₂O₃ isan example of the class of chemicals identified as synthetic pyrethroidsand other examples are deltamethrin, sypermethrin, allethrin andbifenthrin. If a house dust mite comes in to contact with a sufficientconcentration of permethrin, the mite is killed and ultimately a colonyof mites would be destroyed. Furthermore, weaker concentrations ofpermethrin can be detected by mites and as such the mites are deterredfrom initiating an infestation.

[0031] Further investigation on the part of the present applicants hasbeen initiated, given that trials conducted using netting impregnatedwith permethrin provided protection against house dust mites for aperiod of time considerably less than that anticipated as a result oflaboratory trials. Further investigation has shown that although acovering may contain sufficient permethrin to deter or even kill housedust mites, as concentrations decrease, house dust mites may still beattracted to surrounding soft furnishing. Thus, although the presence ofa mattress cover may prevent house dust mites from coming close to thecover itself, as concentrations of permethrin in the cover are reduced,house dust mites may still invade the central core of the mattress andtheir debris may then result in allergenic reactions.

[0032] In laboratory conditions, a covering, possibly fabricated fromnetting, will retain permethrin within the interstices of the net withminimal permethrin loss. However, in a real domestic environment,personal movement results in the material being rubbed, sometimesagainst itself and sometimes against other soft furnishings, such assheets, within the environment. This rubbing results in the permethrinbeing lost from the covering and as a result, over a period of time, thelevel of protection provided by the covering, reduces.

[0033] It may be thought that, in order to reproduce an environmentsimilar to that in a laboratory, it would be preferable to provide asystem in which the permethrin is permanently held within an appropriatecovering such that its concentration does not decrease over a period oftime due to physical movement. However, further investigations haveshown that the release of permethrin from the covering and into thesurrounding environment is an important factor in terms of destroyingand resisting house dust mite infestation.

[0034] As permethrin is lost from the covering, due to physicalmovement, a quantity of permethrin will be absorbed by the mattressitself, thereby providing a level of protection through to the mattresscore in addition to providing a level protection merely at theperiphery. Thus, if a material is used which strongly resists therelease of permethrin, even in response to vigorous physical movement,protection is provided for a long period of time but the degree ofprotection is reduced. Furthermore, increasing the initial dose ofpermethrin to the covering does not significantly improve the situation,given that permethrin is still retained by the covering and does notallow a significant degree of the insecticide to penetrate the mattressor other soft furnishing as a whole.

[0035] The present invention provides an improvement over previouslyknown coverings impregnated with insecticide, in which the insecticideis released from the covering over an operational period so as toencourage the absorption of the insecticide into the soft furnishing. Inorder to achieve the desired degree of absorption, the covering ismanufactured from a machined fabric having interstices sufficientlysmall to reduce the rate of insecticide release over the operationalperiod. In particular, a covering is used that has interstices that aresignificantly smaller than that provided by machined netting. In apreferred covering, a knitted voile fabric is used of the type shown inFIG. 3.

[0036] The knitted voile shown in FIG. 3 differs from conventionalcurtain voile, of the type used for decorative effect. The knitted voileshown in FIG. 3 is slightly denser, that is to say, less transparent andmore stable, than conventional curtain voile although it is made on awarp knitting machine of similar construction to that used for theproduction of curtain voile. However, whereas the production of curtainvoile requires three guide bars, the construction shown in FIG. 3requires only two guide bars. It is particularly suitable for thepurpose proposed due to its larger fibre surface area, resulting ininterstices of a preferred size for retaining the permethrin for a longoperational period, when compared to netting, while at the same timeallowing a degree of permethrin release when stimulated by physicalactivity. Furthermore, the knitted voile presents interstices of morecomplex shape, when compared to netting, further enhancing itsproperties in terms of holding permethrin while allowing a controlledrelease of the insecticide over a period of time.

[0037] Voile of the type shown in FIG. 3 and similar type voiles usedfor curtaining etc are described in “Knitting Technology” by David J.Spencer published by Woodhead Publishing Limited (ISBN 1 85573 3137).

[0038] The knitted voile shown in FIG. 3 shall be identified as PHCvoile, to distinguish it from the type preferably used in themanufacture of curtains. It is knitted from continuous filamentpolyester, ensuring that the permethrin is held between the knittedyarns and is not absorbed by the yarns themselves. When knitted toprovide a similar feel to an equivalent netting, the size of the yarnitself is relatively smaller but the actual knitting pattern, as shownin FIG. 3, is considerably more complex such that, once knitted, theoverall weight per unit area and hence the overall quantity of polyesterwithin the cover, is substantially similar.

[0039] A mattress cover of the type shown in FIG. 2, fabricated from aPHC voile of the type shown in FIG. 3, should provide protection againsthouse dust mites and other insects for a period of two years. The coveris designed to provide protection for a period of two years, given thata user would normally wish to replace such a covering after a period oftime, thus allowing the original dose of permethrin to be keptrelatively modest and thereby harmless to people coming into contactwith the covering. Over this period of two years, dust mites in thevicinity will be killed and larger insects will tend to be repelled bythe presence of the permethrin, which in itself would eliminate thepossibility of an article becoming infested. However, any insects cominginto contact with the cover directly would be killed.

[0040] In use, abrasion will cause permethrin to be lost from thecovering, thereby allowing the insecticide to steep into the mattresscovering 23 and into top layers of mattress wadding 22. In this way, areservoir of permethrin is provided within the mattress itself, therebyextending the range over which mites are killed and other insects arerepelled. Thus, over a preferred operational period of two years, theloss of permethrin from the covering in this way enhances the overalleffectiveness of the covering and the PHC voile ensures that the rate ofpermethrin loss is optimised, as illustrated in FIGS. 4A, 4B and 4C.

[0041] Each of the three graphs shown in FIGS. 4A to 4C illustrate theloss of permethrin from respective cover types over a preferredoperational period of two years. Each covering is provided with asubstantially similar dose of permethrin, identified as one hundredpercent, which, over the operational period, is lost from a mattresscover, primarily through steeping into the underlying mattress.

[0042]FIG. 4A shows a situation in which a conventional nettingmaterial, such as sandfly netting, is used, as suggested in thepreviously identified international patent publication. The loss ofpermethrin substantially follows an exponential decay 401 and this lossof permethrin over the operational period results in a build-up ofpermethrin within the underlying mattress, indicated by curve 402. Curve401 drops off sharply such that the degree of permethrin availablewithin the environment has reduced significantly after six months ofuse. The degree of permethrin available within the mattress, by thesteeping progress, increases rapidly over an initial period and reachesa peak 403 after about four months. Thereafter, as the rate ofpermethrin loss from the cover tails off, the insecticide is lost fromthe underlying mattress at a rate greater than that by which it isreceived from the cover, such that, as illustrated by curve 402, theamount of permethrin within the mattress decreases. Thus, experimentshave shown that in order for the level of permethrin contained withinthe mattress to remain substantially constant, a continual steeping orfeeding from the overlying cover is required over a period of time.

[0043] Conventional systems have lost most of their permethrin from thecover after six months of use but continue to provide protection as aresult of permethrin retained within the mattress itself. However, aftera one year period, much of the permethrin has been lost from themattress and after eighteen months virtually all of the permethrin hasbeen lost, such that over the period from eighteen months to two years,virtually no protection is available. Using a covering of this type, itwould be necessary to replace the covering at one yearly intervals inorder to provide continuous protection. Furthermore, a substantialproportion of the permethrin is lost over an initial two month periodwhich could result in ambient concentrations of the chemical that arehigher than preferred maximums.

[0044] As the permethrin is absorbed by the mattress, its concentrationis diluted. Thus, the permethrin continues to steep through the mattressbut ultimately if the mattress holding permethrin is not continuallytopped up, the overall concentration will drop below an effective level.Furthermore, permethrin breakdown occurs due to natural processes suchthat a continual supply of the chemical is required in order to achievethe pesticide effect.

[0045] The situation for the PHC voile, shown in FIG. 3, is illustratedin FIG. 4B. PHC voile is provided with a substantially similarconcentration but, as illustrated by curve 411, the rate of permethrinloss is significantly less such that a significant degree of permethrinis still available within the cover after the two year operationalperiod. However, as required, there is a loss of permethrin from thecover resulting in a build-up of permethrin within the underlyingmattress. Mattress concentrations are illustrated by curve 412 which, asshown, reaches a maximum level after about four months whereafter theconcentration remains substantially constant just before the two yearoperational end. It can therefore be seen that by using PHC voileinstead of netting, the rate of loss from the cover balances the rate ofloss from the mattress such that, over most of the operational period,the mattress concentration remains substantially constant. Furthermore,the slower permethrin loss ensures that ambient concentrations areminimised, thus avoiding a permethrin high point as illustrated by peak403 in FIG. 4A.

[0046] The PHC voile provides interstices sufficiently small to reducethe rate of insecticide release over the operational period. Othermaterials may be employed which have even smaller interstices, such thatthe insecticide is held more firmly by the material, resulting in aneven slower insecticide release rate over the operational period. Insome circumstances, such a material may be considered preferable if alower rate of permethrin loss is required. However, as illustrated inFIG. 4C, this may not always be desirable, given that it may result inthe concentration of permethrin held by co-operating structures, such asmattress 12, failing to reach optimum concentrations.

[0047] The response shown in FIG. 4C was developed from the use of areverse lock-knit fabric or similar close-knitted fabric as described in“Knitting Technology”. The reverse lock-knit fabric receives asubstantially similar concentration of permethrin to the sandfly netshown in FIG. 4A and to the PHC voile shown in FIG. 4B. Under similaroperational conditions, the rate of permethrin loss is significantlyreduced, as illustrated by curve 421. Although the rate of loss isreduced, there is some permethrin loss, resulting in permethrinconcentrations within the mattress 12 increasing, as illustrated bycurve 422. However, if it is assumed that a concentration of permethrinto the extent defined by curve 412 is required, in order to provideoptimum effect, it can be seen that the concentration provided by curve422 fails to reach the required extent and as such the overallperformance of the material is less than ideal.

[0048] If it is assume that, in accordance with standard hygienepractice, the cover is replaced at two yearly intervals, a significantproportion of the permethrin would be retained within the cover andtherefore not used for the purpose for which it was intended. Thus, inmany systems there is an optimum balance, requiring a material whichretains the permethrin to a degree while allowing a predeterminedrelease rate so as to ensure that permethrin is present within the coveritself over the operational period while at the same time allowing adegree of permethrin to build-up in surrounding structures over theoperational period, such that house dust mites and similar insects arekilled by the cover itself and are repelled from the surroundingstructures.

[0049] Many material structures are known and, in the production of theoverall configuration, many variables may be adjusted in order toachieve the required result. The essential feature of the presentinvention is that the fabric is machined so as to have intersticessufficiently small to reduce the rate of insecticide release over adefined operational period. In the present preferred embodiment, thecover is required to remain operational for at least a period of twoyears and the rate of release is required to be sufficient so as toprovide a sufficient concentration of permethrin within an underlyingmattress.

[0050] In addition to being released into mattresses, the permethrinwould also be released into overlying sheets. Permethrin concentrationin sheets provides additional protection but given that the sheets wouldtend to be changed regularly, it is not possible for significant levelsof the insecticide to build-up to an extent similar to the build-up ofinsecticide in the mattress.

EXAMPLE (MATTRESS COVER)

[0051] Yarn type: Continuous filament polyester thirty-three decitexFabric Type: Warp knitted PHC voile (FIG. 3) Knitting machine type:Twenty-eight gauge tricot warp knitting machine with two guide bars,both fully threaded. Yard feed run-in front guide bar six hundred andsixty-five millimetres per rack back guide bar, nine hundred andninety-eight millimetres per rack. Pattern chain construction frontguide bar 1-0/0-1/ and back guide bar 0-0/3-3/ Knitted quality coursesTwenty nine point three per centimetre: Whales per centimetre: 11.0Construction: Front guide bar chain stitch on one needle Back guide barlaying in over three needles. Resulting fabric weight: Forty-one gramsper square metre.

[0052] PHC voile is knitted as specified above and then tailored intoappropriate coverings for mattresses, pillows or other soft furnishings.The tailored coverings are then processed to provide five hundred andfifty milligrams of permethrin per square metre, and field trials haveshown that a mattress cover of the type shown in FIG. 2 continues to befully operational until a concentration of one hundred and seventy-sixmilligrams per square metre has been reached. Thus, the PHC voile isconstructed to provide a rate of permethrin loss such that, over theoperational period of two years, the permethrin concentration hasreduced from five hundred and fifty milligrams per square metre toapproximately one hundred and seventy-six milligrams per square metre.

[0053] The permethrin impregnation process is illustrated in FIG. 5.Fabricated articles are placed into a feeder 501 such that, in acontrolled manner, the articles are introduced to an impregnation bath502. After passing through the impregnation bath, the articles aretransported through a set of rollers 503 to remove excess impregnationfluid, which is returned to the impregnation bath by a fluid return 504.After being rolled by rollers 503, the articles move into a continuousdryer 505, configured to dry the articles such that solvent is removedfrom the fibres and the permethrin insecticide is retained by the fibresand held by the interstices of the material. From the dryer 505, thearticles are transferred to a store 506, arranged in a controlledtemperature and humidity environment.

[0054] Impregnation bath 502 is constantly stirred with low sheercontinuous paddle stirrers to maintain an even distribution ofpermethrin within the solution. The temperature of dryer 505 should notexceed one hundred and ten degrees Celsius and is preferably maintainedat an optimum working temperature of eighty degrees Celsius.

[0055] Permethrin, also identified as FMC 33297 and by several otherchemical names, is present as two isomers, the relative concentrationsof which would be specified in any particular batch. Permethrin ishydrophobic and does not dissolve in water but can be suspended in wateras an emulsion. To facilitate its suspension in water, permethrin isusually obtained in organic solution, with surfactants; an example beingsold under the trade mark Dragnet having a permethrin concentration ofthirty-six point eight percent.

[0056] The detailed example described herein makes reference to the useof the organic solution distributed under the name “Dragnet”. Analternative solution is distributed by “AgrEvo” under the trade markPermanone. Permanone is available with permethrin concentrations offorty percent or ten percent, thus, when formulating a suspension fordipping purposes, water concentrations should be modified accordingly inorder to produce a fluid of the required permethrin concentration.

[0057] It is possible for articles impregnated by the process shown inFIG. 5 to be recycled. Thus, returned fabrics may be applied to feed 501and a re-impregnation process may be performed, substantially similar toan initial impregnation process.

[0058] Covers such as mattress covers are preferably configured entirelyfrom the PHC voile. However, particularly in alternative configurations,the cover may include portions fabricated from other materials and thecombination, including partial PHC voile and partial non-optimum fabric,may be applied to the process shown in FIG. 5. Composite materials ofthis type may be fully impregnated by the process shown in FIG. 5 or,alternatively, only portions of the composite may be impregnated. Anencapsulating covering may be applied to the composite so as to protectit from the impregnation process in portions where such an impregnationis not required and would be considered undesirable.

[0059] The concentration of permethrin retained by coverings fabricatedfrom machine knitted PHC voile may be controlled by two factors.Firstly, it is determined by the concentration of the chemical in theimpregnation bath 502 and secondly it is determined by the amount ofliquid retained by the fabric prior to being applied to dryer 505. Thus,the mangling operation performed by rollers 503 is an important aspectof the overall procedure, in order to obtain the required level ofchemical concentration. The procedure of wetting the material followedby a process of removing surplus solution is referred to as wadding andthe pressure applied by rollers 503 controls the amount of moistureretained in the textile material, usually referred to as the wetpick-up. Thereafter, the active chemical is retained within the fabricwhile its carrying liquid is evaporated during the drying process.

[0060] Wet pick-up is usually expressed as a percentage of the drytextile weight, such that the percentage wet pick-up equals the weightof liquid retained after impregnation divided by the dry weight andmultiplied by one hundred. In this way, the loading of the chemicalproduct (permethrin in this example) from the impregnating liquordepends upon the relationship between the wet pick-up and the liquorconcentration.

[0061] It is usual to operate impregnation equipment to provide aconsistent and known wet pick-up for any given textile product type.Different chemical loading can then be obtained by varying theconcentration of the chemical agent in the impregnating liquor.

[0062] Polyester fibres are not capable of absorbing significant amountsof water, therefore the aqueous liquor retained by the fabric is presentas a surface layer. This in turn ensures that the permethrin is heldbetween the interstices of the weave and not within the polyester fibresthemselves.

[0063] The preferred permethrin loading is five hundred and fiftymilligrams per square metre and this is achieved by means of a wetpick-up of sixty-five percent using an aqueous dispersion containingsixty-four millilitres of Dragnet concentrate per litre of water.

What we claim is:
 1. A covering for soft furnishing, impregnated with aninsecticide wherein said insecticide is released from said covering overan operational period so as to encourage the absorption of saidinsecticide into said soft furnishing, comprising a coveringmanufactured from machined fabric having interstices sufficiently smallto reduce the rate of insecticide release over said operational period.2. A covering according to claim 1, configured to be applied tomattresses, duvets, pillows cushions, pet beds, or other beddingelements.
 3. A covering according to claim 1, wherein said insecticideis a synthetic pyrethroid.
 4. A covering according to claim 3, whereinsaid synthetic pyrethroid is permethrin, deltamethrin, sypermethrin,allethrin or bifenthrin.
 5. A covering according to claim 1, whereinsaid fabric is machined from a polyester yarn.
 6. A covering accordingto claim 5, wherein said polyester yarn has a gage in the range oftwenty to one hundred decitex.
 7. A covering according to claim 1,wherein the interstices of the fabric are smaller than that provided bya machined netting.
 8. A covering according to claim 7, wherein saidmachined fabric is a knitted voile.
 9. A covering according to claim 8,wherein said knitted voile is machined on a warp knitting machine withtwo guide bars to provide a voile denser than curtain voile.
 10. Acovering according to claim 8 or claim 9, wherein said voile has betweentwenty-five to thirty-five courses per centimetre and between eight tofifteen whales per centimetre.
 11. A covering according to claim 8,claim 9 or claim 10, wherein the fabric has a weight of betweenthirty-five to forty-five grams per square metre.
 12. A coveringaccording to claim 1, containing between four hundred to seven hundredmilligrams of insecticide per square metre.
 13. A covering for softfurnishings, impregnated with permethrin with a loading of between fivehundred and six hundred milligrams per square metre and configured torelease permethrin over an operational period of between one-and-a-halfand two-and-a-half years to give a final loading of between one hundredto two hundred milligrams per square metre.
 14. A covering according toclaim 13, fabricated totally or in part of a knitted voile.
 15. Acovering according to claim 14, wherein said voile is knitted frompolyester yarn.
 16. A cover according to claim 15, wherein saidpolyester yarn has a gauge in the range of twenty to one hundreddecitex.
 17. A method of producing a covering for soft furnishingsconfigured to provide protection against house dust mites, comprisingthe steps of knitting a voile from a polymer yam; machining a cover fromsaid knitted voile; and impregnating said machined cover with aninsecticide.
 18. A method according to claim 17, wherein said polymeryarn is polyester yam having a gauge of between twenty to one hundreddecitex.
 19. A method according to claim 17, wherein said polymer yarnis knitted on a warp knitting machine with two guide bars.
 20. A methodaccording to claim 17, wherein said knitted voile has a weight ofbetween twenty-five to forty-five grams per square metre.
 21. A methodaccording to claim 17, wherein said machined cover is impregnated bybeing immersed in an impregnation fluid and then rolled between rollersto control the level of fluid pick-up.
 22. A method according to claim21, wherein said rolled voile is passed through drying means to removewater from said insecticide.
 23. A method according to claim 21, whereinsaid impregnation fluid is a suspension of permethrin in water.
 24. Amethod according to claim 23, wherein said impregnation fluid is mixedby adding permethrin in organic solution to water to provide saidsuspension of permethrin in water.
 25. A method according to claim 24,wherein said organic solution contains from fife to fifty percentpermethrin.
 26. A method according to claim 24, wherein between fiftyand ninety millilitres of organic solution are added to each litre ofwater.
 27. A method according to claim 21, wherein an insecticideloading of between four hundred and seven hundred milligrams per squaremetre is achieved by a wet pick-up of between fifty to eighty percent.28. A method according to claim 27, wherein said insecticide ispermethrin.